Generator synchronization procedure in Power Plant

Synchronization of Generator with grid power supply is the process of equating the parameters like magnitude, phase and frequency of the both power sources and connecting the generator with the power system network. The power generating plant should be connected to grid power system network to distribute the generated power or to receive power during start-up or on generator trip. Here we will concentrate on delivering power to the grid during generation. To do this the generator has to be connected to the grid power supply with proper synchronization or otherwise the effects of the poor synchronization may leads to serve damage. There are two types of generator synchronization procedures available in the power plant i.e. Manual synchronization and Automatic Synchronization.

 

Manual Synchronization of Generator in power plant

The Manual term in the Manual synchronization indicates that the whole process of synchronization is done by the plant operator. Here the operator is a judge who decides when to close the circuit breaker after observing the equitable parameters like phase; frequency and magnitude are in limits. The following steps indicate the total procedure in the hierarchy.

  1. The synchronization procedure starts when the speed of the Turbo-Generator (TG) reaches to 95% of its rated speed.
  2. The synchronization can be done by the Main Control Room (MCR) panels or Generator control panel (GCP) as per the operator choice.
  3. The Operator selects MAN synchronization from a two state switch called AUTO/MAN synchronization.
  4. Now the synchronization unit on the panel will be on and ready to take input signals.
  5. The operator has to select the circuit breaker to be synchronized from MCR/GCP. Corresponding incoming and running voltages will be selected and fed to the predetermined synchronization check logic.
  6. The Magnitude and frequencies of the two incoming and running frequencies are displayed in the synchronization panel HMI (Human Machine Interface) for operator reference.
  7. The frequency difference is calculated and provided as an input to the synchroscope (Needle type of rotating led lights type) and the needle rotates according to the frequency difference.
  8. The position of the needle is determined by the phase angle difference of the two signals.
  9. At first the operator raises/lowers the generator voltage magnitude by operating the AVR raise/lower push buttons from MCR/GCP to match with grid voltage (i.e. equal or slightly greater).
  10. Once the magnitude matches then the operator adjusts the Speed (Frequency) of the TG by operating the speed raise/lower buttons from the MCR/GCP. This implies controlling the steam flow through the TG by the governing unit of TG. The incoming frequency should be slightly more than the grid frequency because the pull-down force is less hazardous compared to pull-up force after synchronization.
  11. Now the needle of synchroscope rotates in clock wise direction with less RPM. It continuously touches the 12’o clock position. This 12’o clock position indicates that two signals are in phase.
  12. The synchronization check relay provides the permission to close the circuit breaker when the needle is +- 0.5 from 12 o clock position.
  13. The operator initiates the closing of circuit breaker from MCR/GCP when the needle approaches 12 o clock positions, by keeping closing time of circuit breaker in mind and phase angle variation rate.
  14. Once the breaker has been closed, the TG power is raised to house load (block load) by the governing unit (Electro Hydraulic/Digital Hydraulic). In the earlier days the hydraulic speeder gear system also brings the TG to feed block load after synchronization with the speed bias for the load.
  15. The generator power will be raised to full power by the load control unit of modern governors or by adjusting the speed droop characteristics of earlier hydraulic governors.

Advantages of Manual synchronization

  • Here the operator is the judge and at any time closing of the circuit breaker can be delayed if any problem occurs in some other circuit.
  • If any condition is not matched and if it is not predominant for synchronization i.e. the damage is less than the operator can bypasses it and closes the circuit breaker.
  • Failure of computer system during synchronization can be avoided.

Disadvantages of Manual synchronization

  • Closing time of the breaker may not be that much precise which can create damage to the plant equipment.
  • There is possibility of parallax error in adjusting magnitude and frequency.

Automatic Synchronization of Generator in power plant

In growing control system automation, synchronization of generator using automatic control system fetches a lot of advantages over manual synchronization. Here the computer plays the role of the operator which precisely judges the closing of the circuit breaker with predetermined synchronization logic and limit parameters. An Auto- synchronizer cabinet is placed in the GCP with corresponding hardware and software which is connected to ATRS (Automatic Turbine Run-up System), Governing Unit and AVR (Automatic Voltage Regulator) of TG. The typical schematic of arrangement is shown in below figure.


Auto synchronizer unit receives the input from MCR/GCP, two potential meters and provides a four output digital pulse signals to ATRS, Closing signal to the circuit breaker. The four digital pulses are speed and voltage adjustment signals which are provided as inputs to governing unit and AVR through ATRS. The following are the steps in Auto synchronization of Generator in power plant

 

  1. Auto synchronizing mode is selected from MCR/GCP by the operator with the addition of circuit breaker selection.
  2. Once both are selected the Auto synchronizer unit takes the inputs from corresponding logic according to predetermined logic for the circuit breaker. The same readings are passed to the HMI also.
  3. Initially it checks the voltage difference and compares with design difference limit which is already fed to it. If the difference is beyond the limit it sends an output digital raise/lower pulse to the AVR through the ATRS. The pulse duration is depends on difference.
  4. Parallelly it also checks the frequency difference 

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